Galvanic imaging devices utilizing impedance measurements in oil-base mud are very sensitive to the oil-filled gap between the electrode surface and the geologic formation. Multiple attempts have been made to eliminate the problem and are known in the prior art.
Typically, an imaging tool contains a series of small buttons mounted on a metal pad and separated by narrow insulating gaps. The buttons serve as electrically conducting electrodes. In oil-base mud, the measured impedance of individual buttons depends to a large extent on the mud cake parameters. In addition, an oil film on the pad surface may completely eliminate the electrical contact between pad and formation.
In conductive mud, the mud cake has low resistivity and, consequently, is almost transparent to the current flowing through it. In oil-base mud, the mud cake is very resistive, contributing greatly to the measured ground resistance. Therefore, the true value of formation resistivity is significantly obscured. In addition, a thin oil film may cover the surface of the pad, making the overall ground resistance so large that it is practically impossible to inject sufficient current into the formation.
The size of a button is associated with the tool spatial resolution. Usually, the button radius is in the range of 1 to 2 mm, creating a very large ground resistance. For example, a 2 mm button on a typical focusing pad has a ground resistance of 10,000 Ohm in a 1 Ohm-m formation or 10,000,000 Ohm in a 1,000 Ohm-m formation. This illustrates the technical challenge of producing a high definition image in a resistive-mud environment.
In the present invention, a new principle is introduced based on phase sensitive detection with the phase established with respect to a floating reference. The floating reference represents the electric field in the gap. Mud-filled chambers in front of the electrodes are used to measure the reference field.